This is possible with the right device. Assuming you have one, you can do it with the following steps.
Open the home app.
Hit the plus button at the top right.
Choose Automation.
Give it a name, like "Music Automation" or something like that.
Then choose your starter. Choose device, then from the list, choose your PIR sensor.
Once you have chosen it, you'll get some options. For your device you want to choose the occupant option, then "occupied". If you've done it right, it'll be highlighted in the list. You might have other options, but for this part of the automation you can just click done.
You'll be back on the first page of the automation now. You don't need to set a condition, but if you'd like to set one, such as only having the automation work at certain times of day, you can hit the plus button and add a condition.
Once you've done that, add the Action. Hit the plus, and when the list comes up, choose "Ask Google".
At the top, type in your command. Treat this as if you've just said "ok Google", so in the command, you could type "play music" or "play songs by Sia". Then, choose the device from the list that you would like the command to complete on. (The speaker or display that should play the music). Then hit Done.
You'll be able to see the main page for the automation now, if it looks good, hit save at the top right.
You're all set, the next time your sensor changes from Unoccupied to Occupied, the "play music" command will run.
Pete.
